Avalanche Forecast

Issued: Mar 21st, 2017 4:16PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Persistent Slabs.

mgrist, Avalanche Canada

North Columbia.

We've weathered the storm but we're not out of the woods yet. There's a great blog post on critical factors to watch out for this spring. Click here for details.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

We're looking at classic unsettled spring weather for the forecast period: cloudy with isolated (sometimes intense) flurries.WEDNESDAY: Broken skies with isolated flurries (local amounts 5-10cm possible) / moderate south wind /Freezing level 1600mTHURSDAY: Flurries in the morning with 5cm possible / Light east wind / Freezing level 1300mFRIDAY: Isolated flurries (local amounts 5-10cm possible) / Light to moderate southwest wind / Freezing level 1500m

Avalanche Summary

We had a widespread natural avalanche cycle (to Size 4.5) at all elevations and aspects during the recent storm, including multiple adjacent avalanche paths running simultaneously with 1 Km wide propagation.Many of these avalanches ran in the storm snow with some also stepping down to persistent weaknesses (see snowpack summary below) deeper in the snowpack. Of particular concern are the ongoing large destructive avalanches (some scrubbing to ground) that have been occurring daily: one or two deep spooky avalanches in each bulletin region of the Columbia Mountains. See the link in the headline for detailed discussion of these low probability, high consequence events.

Snowpack Summary

We had heavy wet snow (or rain up to 2000m in places) and strong to extreme southerly winds during the weekend's storm. Temperatures also warmed up significantly. The end result: Widespread storm slabs and wind slabs at treeline and above, with significant cornice growth as well.The most recent storm snow (totals of 30-50cm) sits on older windslabs (or soft slabs) at treeline and above. Below 1800m, the new snow (with a breakable surface crust) sits on a melt-freeze crust from rain events last weekend. Reports so far are that the new snow is bonding well to the old crust, but that the storm snow instabilities (down 25 and 45cm) are showing some reactivity to rider traffic.Approximately 100-140 cm below the surface you may find the late-February persistent weakness / crust interface. This layer has woken up from time to time as smaller avalanches still have to potential to 'step down' and trigger this layer.The deep mid-December facet layer (and November raincrust) still linger at the bottom of the snowpack and are the suspected culprit (running to ground/glacier) in Glacier National Park avalanches. See here for one spooky picture. See here for another.

Problems

Wind Slabs

Look for changes in the snow as you climb up through the elevation bands - shooting cracks, denser snow, hollow sounds and cornices above are all good indicators of wind loading.

If triggered, the wind slabs may step down to deeper layers resulting in large avalanches.Use caution in lee areas. Recent wind loading has created wind slabs.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

Persistent weakness layered throughout the snowpack create the potential for very large step-down avalanches. Heavy loads such as a smaller storm/wind slab avalanche or even a cornice failure will increase the likelihood of triggering these layers.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Be aware of the potential for wide propagations.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 4

Valid until: Mar 22nd, 2017 2:00PM